1. Field of the Invention
The present invention relates to a wet-type multiple disc clutch which is used in an automatic transmission or the like of a car.
2. Related Background Art
An automatic transmission of a car generally comprises a planetary gear shifting mechanism of three to five speed levels, in addition to a torque converter serving as a fluid joint, and the speed is changed by properly fixing or releasing a constituent element such as a sun gear or a planetary gear of the planetary gear shifting mechanism by the use of frictional engagement means such as a clutch or a brake. As the frictional engagement means to be built in the automatic transmission, a wet-type multiple disc clutch having a clutch plate set in which friction plates and separator plates are alternately provided is employed except in some band-type brakes. On the other hand, an oil pressure piston which is driven by a pressure oil from a transmission control oil pressure circuit is used for pressure contact (frictional engagement) of the clutch plate set.
In order to release the clutch plate set in the wet-type multiple disc clutch, it is required to discharge an operation oil from a cylinder chamber which drives the oil pressure piston. For this reason, a return spring such as a compression coil spring or a coned disc spring is often interposed between the clutch case and the oil pressure piston, thereby urging the oil pressure piston to the cylinder chamber side all the time. However, since the clutch case is generally rotated at high speed, an oil pressure (centrifugal oil pressure) is generated on the operation oil inside the cylinder chamber due to the centrifugal force, so that the operation oil may not be discharged smoothly from the cylinder chamber only with the return spring. In this case, the clutch may not be released swiftly, or the friction plates and the separator plates may not be sufficiently separated from each other, thereby generating a dragging of the clutch.
In order to solve such problems, a centrifugal oil pressure cancel chamber is provided between the oil pressure piston and the clutch case so as to offset the centrifugal oil pressure in the cylinder chamber by the centrifugal oil pressure of the operation oil inside the centrifugal oil pressure cancel chamber (see, for example, Japanese Patent Application Laid-Open No. 2001-241467, P. 4, FIG. 2, and Japanese Patent Application Laid-Open No. Hei 09-060660, P. 3, FIG. 2). The centrifugal oil pressure cancel chamber is generally defined inside the oil pressure piston by a disc-shaped canceler which also serves as a retainer of the return spring. The canceler is detained on an inner tube of the clutch case by a stop ring.
In the wet-type multiple disc clutch described above, the canceler is pressed against the end surface of the stop ring all the time by the spring force of the return spring which is interposed between the canceler and the oil pressure piston. However, in an automatic transmission which is provided with a take-off clutch of a multiple disc wet type, instead of a torque converter, the canceler is sometimes moved toward the cylinder chamber side against the spring force of the return spring by the pressure of a lubricating oil inside the take-off clutch case.
FIG. 6 shows an attachment portion of the canceler in the conventional wet-type multiple disc clutch. As shown in FIG. 6, the canceler 41 is detained by the stop ring 49 which is fitted in an annular groove 47 on the inner tube 13 of the clutch case, and the canceler 41 is provided with a projected portion 51 for preventing a removal or fall of the stop ring 49. In this device, when the clutch is released and the urging force of the return spring (not shown) is weakened, the canceler 41 is moved toward the cylinder chamber side (to the left in the drawing) by the pressure of the lubricating oil inside the take-off clutch case, as shown in FIG. 7, whereby the stop ring 49 is freed from the restriction by the projected portion 51. Consequently, there arise such inconveniences that the size of the stop ring 49 is expanded due to a vibration of the engine or a running vibration, so as to remove or fall out of the annular groove 47, or the canceler 41, the return spring, or the like, detained by the stop ring 49 falls out inside the clutch case, thereby preventing release of the clutch.